This patch makes Page weakable and allows page-less frames to exist.
Page is single-owner, and Frame is multiple-owner, so it's not sound
for Frame to assume its containing Page will stick around for its own
entire lifetime.
Fixes#3976.
Form submissions to file:// URLs are now permitted only if the
submitting document is also a file:// URL and the form method is "get".
Form submissions to URLs with a http(s):// URL protocol are permitted.
Form submissions for all other URL protocols are rejected.
LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
